The present invention relates to a process for encapsulating solutions of reactants of colour reaction systems by means of conventional encapsulation processes, capsules produced according to such a process and the use thereof in non-carbon papers.
In the case of commercial non-carbon papers, such as are described e.g. in German Pat. No. 2,151,178, as well as U.S. Pat. Nos. 3,418,250 and 3,016,308, the colour formers, e.g. in the form of crystal violet lactone, which during the printing process with an acid reactant supply a colour marking, are enclosed in microcapsules. If the acid reactant is also soluble, it can also be encapsulated. Encapsulation is intended to prevent the undesired premature colour reaction of the two reactants. Known colour formers are superior conventional aromatic systems. They are consequently also dissolved in aromatic solvents and undergo an encapsulation process, accompanied by the formation of a relatively hard capsule wall. The known aromatic solvents for aromatic colour formers, such as e.g. alkylated biphenyls, particularly butyl biphenyl, isopropyl biphenyl, amyl biphenyl, hexyl biphenyl and diisopropyl biphenyl, as well as alkylated naphthalenes, terphenyls and their derivatives, particularly partly hydrogenated terphenyls, diarylmethane and its derivatives, as well as diarylethane and its derivatives are relatively difficult to prepare and/or expensive. Numerous attempts have been made to take account of this disadvantage, in that e.g. in the case where isopropyl biphenyl, optionally with a proportion of polyisopropyl biphenyl and biphenyl, is used as a readily dissolving solvent of an aromatic colour former, with the colour former solution obtained is admixed a high-boiling, saturated aliphatic hydrocarbon oil, e.g. with a boiling range of 160.degree. to 288.degree. C. and consequently an inexpensive aliphatic solvent, as the diluent.
The use of aliphatic diluents consequently takes place because they are generally much cheaper than aromatic solvents, such as isopropyl biphenyl. The addition of the diluent reduces the solubility of the colour former in the solvent mixture. If crystal violet lactone is used as the colour former, in view of the practical requirements (adequate solubility, optimum colour formation, etc) isopropyl biphenyl and inexpensive aliphatic hydrocarbon oil can be used in a mixture with a weight ratio of approximately 2:1. Therefore, the inexpensive aliphatic portion represents a maximum of approximately 33% of the total mixture.
Attempts to raise the proportion of the inexpensive aliphatic non-solvent in the mixture with the readily dissolving solvent have failed, because the increase in the non-solvent proportion always led to the dissolved colour former being undesirably at least partly obtained in solid form and consequently the resulting dispersion (with suspended solid particles) was no longer usable for encapsulation by means of conventional encapsulation processes. It was found that if such suspensions were encapsulated, the capsules obtained did not lead to the sought effects in the colour reaction system and in particular did not give the desired colour intensity. Tests revealed that a solution of crystal violet lactone in a mixture of 15% of an aromatic solvent in the form of diisopropyl-naphthalene, partly hydrogenated terphenyl or isopropyl biphenyl and 85% of a non-dissolving aliphatic hydrocarbon led to a substantial precipitation of the colour former within a very short time. However, if capsules for non-carbon papers were produced with such a dispersed medium, it was found that during the subsequent typing process that the precipitated main part of the colour former was withdrawn from the colour reaction mechanism, so that an unsatisfactory colour intensity resulted.
Thus, the attempts to use larger proportions of inexpensive non-aromatic, non-solvents, together with aromatic solvents in the encapsulation of colour former solutions have not been successful according to the prior art. In fact, it was assumed that research in this direction could not be successful.
However, it has surprisingly been found that the desired colour reactions of the colour former and its acid reactant in colour reaction systems take place to the desired extent, if e.g. an inexpensive non-solvent or the colour former and/or its acid reactant is used in a quantity such that even a highly supersaturated solution of the reactants is obtained, provided that that immediately after the preparation of such a super-saturated solution, whilst it is still in fresh form undergoes an emulsification and encapsulation process without delay. If such capsules are used in colour reaction systems, the colour reactions occurring during the printing process are suprisingly unimpaired e.g. in the same way as if a purely non-saturated aromatic system was enclosed in the capsules, instead of a supersaturated system.